Engagement of the multicomponent antigen receptor in T cells (TCR) results in rapid activation of a protein tyrosine kinase pathway. A major TCR- associated protein tyrosine kinase is ZAP-70, a protein that binds to the activated TCR. Under conditions of TCR crosslinking with anti- receptor antibodies, the ZAP-70 bound to the TCR is itself tyrosine phosphorylated and activated. Studies using T cell clones responding to peptide antigens demonstrate that ZAP-70 activation also occurs in that setting. For a given T cell clone, slight alteration of the peptide antigen or the major histocompatibility-encoded presenting molecule can result in partial activation or inhibition. Under these conditions, the TCR is partially tyrosine phosphorylated, and though the ZAP-70 molecule is bound to the TCR, it is neither tyrosine phosphorylated nor active. These studies have great relevance for understanding immunological tolerance as well as T cell activation. Moreover they demonstrate that regulation of ZAP-70 activity is at the level of phosphorylation as well as TCR binding. We have determined the sites of tyrosine phosphorylation of the ZAP-70 kinase. Using site-directed mutagenesis we have mutated two critical tyrosine residues in the kinase domain of ZAP-70, and have demonstrated that one of these sites is required for activation of the enzyme. These studies enhance our understanding of the mechanism of activation of this enzyme. Additional studies focus on a substrate of tyrosine kinases in T cells, p120cbl, a proto-oncogene, which may be involved in the ras pathway in T cells. Recent studies demonstrate that this protein can be found in a complex with the linker protein Grb2. The amount of Grb2-p120cbl complex is regulated by TCR activation. Studies on the dysregulation of tyrosine phosphorylation pathways seen with HIV infection have demonstrated that the tyrosine kinases Lck and Fyn, but not ZAP-70 are activated by HIV gp120 binding to CD4.